Millions of clutches are in common use today in various devices for the purpose of rapidly stopping the rotation of the drive shaft upon the occurrence of some predetermined event, such as upon the deenergization of the motor driving the shaft.
Many of these clutches have used frictional surfaces which frictionally engage each other to absorb the inertia of the rotating drive shaft. When the absorption of this inertia without undue shock is desired, the frictional type clutch is highly desirable since the frictional clutch will decelerate the shaft without inducing much of a shock load to the shaft.
Commonly used friction type clutches fall into two general categories: disc clutches and cone clutches. Each of these types of brakes has certain disadvantages.
The disc type clutch involves two generally flat frictional surfaces which are moved into and out of engagement by appropriate means. In order to provide a large braking torque, the discs, as well as the associated components, are comparatively large. Accordingly, clutches of the disc type are not often used when a compact, lightweight clutch assembly is needed.
On the other hand, the conical type friction clutch provides a pair of frustoconical mating frictional surfaces which are selectively engaged with each other to absorb the inertia of the drive shaft. While the use of a frustoconical surface increases the frictional torque for this type of device, as compared to a disc clutch, clutches of this type are also somewhat large and heavy if they are to exert sufficient frictional torque on the drive shaft to rapidly decelerate the drive shaft.
Furthermore, it is often difficult in a small, compact, and inexpensive frictional clutch of either of the above described types to fully separate the frictional surfaces, thus resulting in a drag torque between the two frictional surfaces during normal operation of the drive shaft. The drag torque results in an increased power consumption by the drive shaft as well as in a rapid deterioration of the frictional surfaces resulting in the need to replace frictional surfaces more rapidly than would be the case if no drag was experienced during normal operation.
An alternate solution to the above described problem is to provide a clutch first gear having face teeth depending from the drive shaft selectively engageable with a second gear movably interconnected with the clutch housing so as to positively stop the drive shaft upon engagement of the teeth. While this configuration does result in a clutch which positively stops the drive shaft, the engagement of the teeth does result in a torsional shock load in the clutch, which load is transferred to the drive shaft. Clearly, then, all the conponents in the clutch assembly must be made strong enough to withstand a sudden elevated torsional load experienced initially during engagement of the teeth.
It would be useful to provide a lightweight, compact and inexpensive clutch combining the advantages of each of the above types of clutches yet avoiding many of their disadvantages. The desired clutch should impart a minimum torsional shock during the clutch application yet should rapidly decelerate the drive shaft. Finally, the desired clutch should not waste power through a large drag torque during normal operation of the drive shaft.